In general, oil or electric power is supplied to an automobile or an electric automobile through a lid box provided in an outer panel (particularly, a side outer panel) of a vehicle body. The lid box is separated from the outer panel of the vehicle body, and for example, Patent Literature 1 below discloses a technique of joining the lid box to the side outer panel by spot-welding.
In the spot-welding, as illustrated in FIG. 6, a welding gun 150 and a back electrode 160 are used, a joining surface of a portion 112 made by bending a side outer panel 110 in the vehicle inner direction is pressed by the welding gun 150 in the direction indicated by the arrow of the drawing, and is spot-welded while completely coming into close contact with a joining surface of an edge 132 of a cup-shaped lid box 130.